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Dynamic Structures and Their Sedimentation Effects in Huangmaohai Estuary, China

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Dynamic Structures and Their Sedimentation Effects in Huangmaohai Estuary, China Journal of Coastal Research 808--820 Fort Lauderdale, Florida Summer 1995 Dynamic Structures and Their Sedimentation Effects in Huangmaohai Estuary, China Chaoyu Wut and Shuyao Yuanf tlnstitute of Coastal and tlnstitute of South China Sea Estuarine Studies Oceanology Zhongshan University Chinese Academy of Science Guangzhou 510275, China Guangzhou , China ABSTRACT _ WU, C. and YUAN, S,,1995. Dynamicstruetures and their sed imsntation effecta in Huangmsohai Estuary Chins. Journal of Coostal R..earch, (1(3) , 8OS-820. Fort Lauderdale (Flor ida) . ISSN 0749·0208. ' Based ,on data c~Uected (rom three cruises of hydrographic surv ey carried out during 19~1991 and num.rI~ modelling the present paper provides a brier analys is on the dynamics of Huangmaohai Estuary, emph88lZlng on the low frequency o( Bow patterns or circulation. Labeled wster parcel trsjectories are caJcu~ated to ~.veal the Lagrangian motio,n. Velocity distribution of the estuary indicates a low snergy zone In the mid estuary that acts 88 a sediment trap. Three large scale dynamic structures are found in tbe estuary along the North and East Channel which have profound effects on the modern sed imentation. They tl1~ (rom north to south (1) vertical de nsity driven circulation, (2) gorge jet current and (3) horbontel CIr~8t10n. The effecte of eetuarine dynamics on the sedimentation process , especially those concerning the river mouth bar, are then discussed. INTRODUCTION ment of Lagrangian motion is in the same order Huangmaohai Estuary is one of the eight out­ of magnitude as tidal excursion. Lagrangian re­ lets of the Pearl River. It is important for various sidual currents, especially their distribution pat­ economic developments of the west Guangdong, terns in the estuary are shown to be responsible China. Several multi-discipline surveys have been to the transport of suspended sediments. carried out since the 1980's. Because its topog­ The density-difference-driven residual cur­ raphy is complicated with channels, shoals and rents play an important role in Huangmaohai Es­ tidal flats are situated alternatively and the fresh tuary as in many estuaries in the world. However, water discharge is subjected to significant season­ in a particular estuary, the local topographical al variations against the unequal semi-diurnal effects may become the dominant process in a tides. Its dynamic characteristics are not well un­ certain area . The Kelvin wave is of importance in derstood. Modern sedimentation encompasses the the Huangmaohai Estuary and in other broad es­ entire complex of interrelated physical, chemical tuaries in South China where long term circula­ and biological processes that bring about the ac­ tion is a concern. cumulation of sediments. Hydrodynamic analysis STUDY AREA that reveals the basic physical process is funda­ mental to a deeper understanding of the modern Huangmaohai Estuary is located in the west of morphological and sedimentational processes in Pearl River Delta. Tanjiang River and Hutiaomen an estuary. River flow in from the north and are the prin ­ Based on field data and numerical modelling cipal sources of fresh water (Figure 1). Geomor­ techniques, in view of Eulerian and Lagrangian phologically, it is a drowned river valley. It is 40 motion, the present research attempts to identify km long and 1.9 km wide in the upper, north end some of the most significant dynamic structures and has maximum width of 34 km near the mouth . that have profound effects on flow patterns and The estuary has a surface area of 548 km", Two long term transport of water and sediments. The sets of rocky island line up near the mouth in the macro-scale properties of the estuarine dynamics orientation of ENE-WSW. The estuary is bathy­ are the main concern. Huangmaohai Estuary is a metrically complex. A scoured deep channel ex­ convection -dominated system . The net displace- tends 20 km from the head towards south which is mainly the result of fresh run -off and strong 94065 received and accepted in revision 7 July 1994, tidal current in the upper estuary. The North Structures and Sedimentation 809 Channel is about 300-900 m wide and 12 m deep. Two channels are found in the lower estuary. West and East Channel are separated by Damang Is­ land. In the mid estuary, North Channel and the Meters XINHUI channels in the south are separated by a broad • APR 1988 shoal, or river mouth bar where the water depth 22 0 .~~.~; @) JAN is only 2 m which is the major obstacle for navi­ 1989 10' :"~"'i$ 0 JUN 1991 gation. Mean tidal range near the head is 1.24 m. r:-,";;I~,:~:Jj Annual mean fresh water discharge from the two "', , rivers is 1,262 m-/sec, 0 5 TIDAL MODELS KM ~ 0 A two dimensional tidal model is applied to ZHUHAI simulate the fluid dynamics of the estuary. Since the actual transport of water and suspended sed­ ~ iment is a Lagrangian process, a numerical ap­ proach is also incorporated to the 2-D model to r<s~.~ simulate the Lagrangian motion. o ~1\ HUANGMAO ~ Governing Equations o • 1112 The large scale horizontal motion in an estuary o H2 can be derived by the shallow water equations: o H3 • 1113 au au au au -+u-+v-+w-at ax ay az f kPaWxlWxl = -g-ar + v + ax pH gu( u2 + V2) O.5 (1) C2H av av av av - + u- + v- + w­ at ax ay az Figure 1. Location map of study area and current meter moor­ ing. = _gar _ fu + kPaWylWyl ay pH (2) C(x, y) is Chezy coefficient f is Coriolis parameter ar a a P, Pa are density of sea water and air re­ - + -(Hu) + -(Hv) = 0 (3) at ax ay spectively k is friction coefficient where are wind speed in x and y directions. u(x, y, t) is depth-averaged velocity in x direc­ Boundary conditions: tion u r = uop(x, y, t) v(x, y, t) is depth-averaged velocity in y direc- V = vop(x, y, t) tion r g is gravitational acceleration where uopand vopare velocities in the open bound­ x is horizontal coordinate (south) ary. For tide controlled boundary: y is horizontal coordinate (east) I', = rop(x, y, t), H(x, y) is water depth, H = h + r h(x, y) is water depth below plane z = 0 In the present model, observed tidal boundary r(x, y) is water elevation above plane z = 0 was used. For closed boundary, Journal of Coastal Research, Vol. 11, No.3, 1995 810 Wu and Yuan HUANGMAOHAI D 5 KM HUANGMAO HAl ESTUARY Figure 2. Map of bottom topography. Figure 3. Flow field, flow direction is tangent to the local iso­ bath. (Wn) = 0 where W is velocity vector and n is the direction vector normal to the boundary. The area of in­ yo) at time to' As time elapses, the position of terest is Huangmaohai Estuary. A North South water parcel can be given by oriented finite difference schematization was made X=x,,+l1 with grid size x = y = 600 m. The shallow-water equations (1) to (3) have been solved by an al­ y = Yo + ~ (4) ternative direction implicit (AD!) finite difference ~ method. Time step of 300 seconds gives enough with the initial conditions that 11 = = 0 at time t = to' Thus the Lagrangian velocity (u., v,) can stability. The model is calibrated so that both calculated tide elevation and current velocities be expressed in terms of the Eulerian tidal veloc­ are compared with observed records with fair ity: agreements. Lagrangian Motion When the transport of dissolved and suspended where the position of the water parcel is (x", Yo) matter is of interest, the actual transport is char­ at t = to' The Lagrangian displacement (11 , ~) can acterized as a convection dominated process which be obtained from integrating the differential suggests that the transport is Lagrangian motion. equation of a streakline: The fate of suspended matter should be derived by a Lagrangian mean rather than by an Eulerian (6) mean (CHENG et al., 1986). In order to analyze the Lagrangian motion of labeled water parcel, the Lagrangian velocity components (u., VI) in the or x, y directions are defined as the velocity of a water parcel at time t which is released from (x", Journal of Coastal Research, Vol. 11, No. 3, 1995 Structures and Sed imentation 811 (A) VELOCITY Cllis TIDAL VELOCITY (e) nDAL VELOCITY JIlJ[: 1J HOUR Figure 4. Velocity isogram in different tidal phases. (D) shows th e residual velocity. •Journal of Coastal Researc h. Vol. 11, No. 3. 1995 812 Wu and Yuan ~, season. This low energy zone acts as a sediment = fto+T [utx, + 1/, Yo + t), trap for both suspended and bed load from inland and marine sources. vtx, + 1], Yo + ~)] dt (7) Lagrangian Motion where T is one or more tidal periods. Eq. (7) is The verified 2-D numerical model is used as the an integral equation of (1], ~). With appropriate based-line flow field and the movement of marked initial and boundary conditions, Eqs. (1) to (7) water parcels can be calculated from Eq. (7). describe completely the dependent variables (r, Shown in Figure 5 are the labeled water parcel u, v, u., V 1], ~). h trajectories computed in a Lagrangian sense over TIDAL CURRENT ANALYSIS a period of diurnal and semi-diurnal tides. La­ grangian net displacement in Huangmaohai Es­ Eulerian Velocity Distribution and its Effects on tuary is smaller than tidal excursion as required Suspended Sediment Transport but it is large enough to be the same order of An important property of the tidal current ve­ magnitude as tidal excursion. As a comparison, locity in the estuary is that the tidal current, both the Lagrangian net displacement is one order of magnitude and direction, is mainly controlled by magnitude smaller than tidal excursion in a semi­ the bathymetry.
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